I propose to you an alternate form of the child's answer to where do babies come from. Instead of the usual stork fairytale or, if you dare, the truth, try this out for size when the big question arises.
There is a little woodland knome that lives in the deepest, darkest, most dense forest that adventurers have ever discovered. This little woodland man has the most important job in the world. He sits on the ground all day blowing into a vine connected to a very large stump. Out of the stump comes bubbles in the shape of animal heads. All day long, one bubble after another. These bubbles float around the forest, a forest covered with other stumps, in the shape of all the animals in the world, searching for the correct combination. When the correct combination happens, the bubble floats down to the stump and a new animal jumps away.
There is a little woodland knome that lives in the deepest, darkest, most dense forest that adventurers have ever discovered. This little woodland man has the most important job in the world. He sits on the ground all day blowing into a vine connected to a very large stump. Out of the stump comes bubbles in the shape of animal heads. All day long, one bubble after another. These bubbles float around the forest, a forest covered with other stumps, in the shape of all the animals in the world, searching for the correct combination. When the correct combination happens, the bubble floats down to the stump and a new animal jumps away.